The majority of miniaturized pumps used on devices such as atomizers for perfume, insecticides and medicines are precompression pumps wherein upon depression the opening of the discharge orifice is delayed due to the relative positioning of the piston, the discharge orifice or evacuation duct and the pump chamber or due to a mechanical resistance. In such a pump, the depression of the piston initially causes an accumulation of pressure or energy in the pump chamber which results in a clear cut and even spraying of the pump chamber contents upon establishing communication between the pump chamber and the discharge orifice. The pressure or energy accumulation in the pump chamber occurs during the first part of the plunging stroke of the piston by means of compression of air or of an auxiliary elastic-return element, and in the second part of the plunging stroke of the piston or when the piston has reached its low point the accumulated pressure or energy causes the even and clear cut spraying of the pump chamber contents.
One of the simplest examples of this type of pump may be referred to in U.S. Pat. No. 3,194,447. In these types of pumps there are very few moving elements, and the piston acts as a slide valve inside the pump body and blocks or unblocks the discharge orifice at mid-stroke. At the beginning of an upward motion under the effect of its return spring, the piston causes air to be drawn into the pump chamber through the discharge orifice. When the piston is forced to descend, the air in the pump chamber is first compressed, and then, upon establishing communication between the pump chamber and the discharge orifice, the compressed air will cause material in the pump chamber to be entrained therein and atomized through the discharge orifice.